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With a single-word response, Senator Joseph R. Biden Jr.surprised and amused his listeners in the first Democratic primary debate, in April 2007. He was asked if he could be disciplined on the world stage and restrain his legendary loquaciousness.

“Yes,” he said.

No one expected Mr. Biden to stop there, but he did, leaving an expectant silence, until the audience caught the joke and burst into laughter.

He showed less restraint in a CNN/YouTube debate a few months later, when a gun owner asked where the candidates stood on gun control, saying he wanted to know if his “babies” would be safe. “This is my baby,” the man said on the video, showing off his Bushmaster AR-15.

“I’ll tell you what,” Mr. Biden replied. “If that is his baby, he needs help.”

The audience applauded enthusiastically, but Mr. Biden did not stop there.

He went on to deride the questioner, saying he incriminated himself because the man said he bought the gun while it was banned, then he questioned the man’s stability. “I don’t know that he is mentally qualified to own that gun,” he said in a gratuitous aside.

The Democrats held 26 debates during the primary season. Mr. Biden, of Delaware, participated in 14 of them before he dropped out of the race Jan. 3, after he came in fifth in the Iowa caucuses. That would seem to give him a huge advantage going into Thursday’s vice-presidential debatewith Gov. Sarah Palinof Alaska, who has never debated on the national stage.

But his off-putting remark to the gun owner suggests that perhaps his “yes” answer to the question about self-discipline had been premature and that there are perils ahead for Mr. Biden on Thursday — both because of his tendency to go too far and the hazards of debating a woman.

A review of Mr. Biden’s debate performances shows him to be deeply knowledgeable across a range of topics, reflecting his nearly four decades in Washington, where he is chairman of the Senate Foreign Relations Committee.

Mr. Biden’s answers tend to gush forth and his voice is raspy, which lends his arguments an air of urgency. He also uses assertive phrases like, “the truth is,” or “folks, let me tell you,” which grab listeners by the lapel.

At the June 3 debate in New Hampshire, for example, he was asked to defend his vote to continue financing the war in Iraq, a vote sought by the White House and criticized by fellow Democrats as an open-ended commitment to the war. All the other Democrats on stage voted against it, including Senator Barack Obamaof Illinois, the presidential nominee who has picked Mr. Biden as his running mate.

“I love these guys who tell you they’re going to stop the war,” Mr. Biden said of his fellow Democrats. “Let me tell you straight up the truth. The truth of the matter is, the only one that’s emboldened the enemy has been George Bush by his policies, not us funding the war.”

One danger for Mr. Biden on Thursday is that his habit of speaking authoritatively, of saying he possesses the truth, will come across as overbearing or condescending, particularly toward someone like Ms. Palin, who lacks his credentials. To try to guard against sounding sexist, he is sparring in practice sessions with Gov. Jennifer M. Granholmof Michigan, who is playing the role of Ms. Palin.

The only other time a woman has appeared on the debate stage as part of a major-party ticket was in 1984, when Geraldine A. Ferraro, the Democratic vice-presidential nominee, faced Vice President George Bush. One exchange might offer Mr. Biden a good lesson.

Slideshow: A look at Biden
Mr. Bush had said, “Let me help you with the difference, Mrs. Ferraro, between Iran and the embassy in Lebanon.” Ms. Ferraro instantly highlighted what she perceived as condescension: “I almost resent, Vice President Bush, your patronizing attitude that you have to teach me about foreign policy.”

Mr. Bush underscored one of the hazards of debating a woman when he later gloated into an open microphone, “We tried to kick a little ass last night.”

The risk may be even greater for Mr. Biden. His innate exuberance and gusto in speaking without stopping for air can make him sound like he is clubbing his points — and his opponent.

He loves railing against the Republicans; he did so most memorably in an October debate in Philadelphia, when he said of former Mayor Rudolph W. Giulianiof New York, “There’s only three things he mentions in a sentence: a noun and a verb and 9/11.”

The line was a huge hit, but again, Mr. Biden did not let it rest. Although the question had nothing to do with Mr. Giuliani, Mr. Biden milked it for 43 seconds more.

Other perils for Mr. Biden are unrelated to Ms. Palin.

He has a tendency to blurt out whatever is on his mind. Even as the vice-presidential nominee, Mr. Biden has veered off script, creating a series of flaps in recent days, from opposing the bailout of the American International Group, which Mr. Obama supported, to labeling as “terrible” an Obama campaign commercial against Senator John McCain, the Republican nominee.

Mr. Biden describes himself as blunt.

He was asked at a Dec. 13 debate in Iowa about whether some of his earlier comments — that Mr. Obama “is articulate and bright and clean,” for example — reflected a discomfort with the subject of race.

“It may be possible because I speak so bluntly that people misunderstand,” Mr. Biden said, defending his commitment to civil rights. Mr. Biden looked sad as Mr. Obama himself stepped in to vouch for him.

One of the consequences of a long time in the Senate is a long record of votes for which one can be held accountable, just as a consequence of a long primary can be a long record of attacks on allies. On Thursday, Mr. Biden may have to answer for both. See the debate of June 3, 2007, for how he twisted himself in knots over a vote for a 700-mile-long fence along the border with Mexico. And see Mr. Biden’s statement that votes against financing the Iraq war by other Democratic candidates, including Mr. Obama, amount to “cutting off support that will save the lives of thousands of American troops.”

Joseph A. Pika, a political scientist at the University of Delawarewho has observed Mr. Biden over much of his career, said the senator was prone to making broad declarations — “We’ve got to level with the American people!” — then expounding with lengthy elaborations.

“He likes to hold forth,” Dr. Pika said. “Being an effective debater will require him to be disciplined and focused and to make his points punchier than is customarily his style.”